This invention relates to an automatic control device for automatically controlling iris diaphragms in optical systems by means of an annular rotary electric motor.
There have been proposed and practically employed a variety of automatic control devices for iris diaphragms and in most of the prior art automatic control devices of the above type, although the rotor formed of a permanent magnet may be an external rotor or inner rotor as desired, the inner rotor has been in most cases employed because the inner rotor is less subjected to the influence of external magnetic force. In such a conventional automatic control device, the rotary electric motor comprises an annular rotor and an annular stator disposed about the rotor in coaxial relationship to the latter. The stator has magnetic pole receiving iron cores extending radially inwardly from the inner surface of the stator core and stator coils are wound about the magnetic pole receiving iron cores (refer to Japanese Utility Model Application Publication No. 45631/1978).
In the conventional automatic control device for iris diaphragms described hereinabove, since the coils are wound about the magnetic pole receiving iron cores projecting radially inwardly from the inner surface of the annular stator core requiring a highly skilled hand and particularly, it is very difficult to manufacture the control device as a small size device. In fact, critical reduction in the outer diameter of the stator core is limited to the order of 40 mm at the best to thereby make it impossible to manufacture the control device as a practically small size device.
And in the conventional automatic control device for iris diaphragms of the above-mentioned type, since the magnetic pole receiving iron cores and the magnetic poles formed on the rotor formed of a permanent magnet face each other directly, in order to satisfy the requirement called for the rotary electric motor for the automatic control device for iris diaphragms, that is, in order that when the rotor ceases its rotation instantly upon the interruption of current supply to the coils and maintains the stopped position precisely, it is required that the magnetic pole receiving iron cores on the stator core are slanted with respect to the axis of the core or the magnetic poles formed on the permanent magnet rotor are slanted with respect to the axis of the rotor so that the sum of the areas of the opposing ends of the magnetic pole receiving iron cores and of the magnetic poles formed on the permanent magnet rotor remains unchanged irrespective of what position the rotor may assume when the rotor ceases its rotation. Such an arrangement is of no practical use and makes it almost impossible to obtain uniform flux density distribution across the entire area of the magnetic poles formed on the permanent magnet rotor. For attaining uniform flux density distribution across the entire area of the magnetic poles, even if the magnetic pole receiving iron cores and magnetic poles are so precisely designed that the sum of the areas of the opposing ends of the iron cores and of the magnetic poles is mechanically maintained unchanged, slight irregularity inevitably occurs in magnetic attraction. Thus, the rotor can not maintain its stationary condition in a particular rotated position resulting in hunting by the electric motor.